I want to tackle my A/C again in the next month or so.
In short, my pressures are too high. 105F Ambient is 55psi / 340psi at 2000rpm. And please, before anybody states it, I am not overcharged. --Anything over 16oz of R134a put me over 300psi.

Q1: Whats the most efficient way to FULLY flush the system to ensure I am starting with a clean slate? Can I flush the evaporator safely once I remove the expansion valve? I have A/C Flush solvent and a compressor. Once I run it through and use the compressor to blow it all out clean, how long should I wait to ensure its completely evaporated off?

Q2: I've only ever vacuumed down for 15-20mins. How "long" is long enough to ensure 100% boil-off of any air in the system so I am 100% sure I have a full vacuum throughout?

Parts I will replace:
Compressor (as I have a brand new Denso one ready)
Expansion Valve
Drier
Main A/C hose (as its leaking slightly).

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2016 Monsoon Gray Audi Allroad - 21k
2008 Black Mercedes E350 4Matic Sport - 131k
2014 Jeep Wranger Unlimited Sahara - 62k
2003 Gray Mercedes ML350 - 122k

Low pressure sounds "about right" if your vents output air ~60 F. The high-pressure sounds too high even for 105 F. I would expect the high-pressure switch would turn off the clutch to prevent that, so insure it is working and that your pressure measurement is correct. If truly 340 psig, you run the risk of the clutch slipping and then quickly melting, which happened to my 1985 300D and 2002 T&C, both on >100 F days. Ensure your condenser fins are clean and that the electric "radiator" fan turns on when it should. As I understand the system, the expansion valve controls the evaporator pressure (and temperature since "saturated" condition), and that spec sounds OK.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans

Looking at your numbers I think the evap and compressor are working fine. What are the conditions under which you see 340/55? Is it 2000rpm with a heat soaked car in the sun? Probably reasonable for those conditions.

I'd be looking at condenser performance. You need to dump more heat to bring the pressures down and get better cooling. Given where you live perhaps hacking together a parallel condenser mod like the 123 guys is the way to go.

About the best performance I ever saw from a 124 is discharge air that is a 30F drop from ambient. So it's important to have the 100%'recirc function working, otherwise you will never see cabin temps where you want them to be.

What compressor does the TE use? My TD wagon with the 10p15 always struggled in Texas heat, but the '93 2.8 sedan with a 10p17 and factory 134a was almost uncomfortably cold in the same conditions.

124 is a newer model than 123 with a "better" compressor. Discharge air that is a 30F drop from ambient does not sound right. I have a stock system with R12 (with R4 compressor) in my 85 300D and I can get discharge air with a 40 F drop from ambient at idle. At cruise should get even better than that.

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Looking at your numbers I think the evap and compressor are working fine. What are the conditions under which you see 340/55? Is it 2000rpm with a heat soaked car in the sun? Probably reasonable for those conditions.

I'd be looking at condenser performance. You need to dump more heat to bring the pressures down and get better cooling. Given where you live perhaps hacking together a parallel condenser mod like the 123 guys is the way to go.

About the best performance I ever saw from a 124 is discharge air that is a 30F drop from ambient. So it's important to have the 100%'recirc function working, otherwise you will never see cabin temps where you want them to be.

What compressor does the TE use? My TD wagon with the 10p15 always struggled in Texas heat, but the '93 2.8 sedan with a 10p17 and factory 134a was almost uncomfortably cold in the same conditions.

Pump down for about one hour. I have always used a micron gauge. I can usually pump down and get a somewhat stable 1000 micron reading. Then you know definitively you are tight and dry.

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Eugene

10 E63 AMG
93 300te 4matic
07 BMW X3
14 Ford F-150 Fx2

Only way to be 100% free of moisture on mobile a.c. systems is with nitrogen purging 100% of time while components are replaced or cleaned, and flow nitrogen complete blow out... then PRESSURE TEST WITH NITROGEN FOR 24 HOURS to be certain of a leak free system...

Vacuuming will never verify water boil off... if you are leaking.

A digital micron gauge holding below 1000 microns 24 HOURS would do the same...

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 560SL convertible
1987 190D 2.5-5SPEED!!!

1987 300TD
2005 Dodge Sprinter 2500 158"WB
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

I agree. Sweep with nitrogen and then pressurize it to about 125psi. Wait overnight and see if it drops at all.

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Eugene

10 E63 AMG
93 300te 4matic
07 BMW X3
14 Ford F-150 Fx2

Yup. There's a reason why "mission critical" systems (commercial marine/ Navy/ clean rooms/ etc) require a triple evacuation- Vac down to under 1000 microns, nitrogen sweep, pressure test. Then vac down again (this time you should see under 500 microns), then nitrogen sweep. Vac down a final time (hopefully you can get under 200 microns here), and then let the charge in. Doing this you will assure that you truly have a dry system (and no leaks).

Nobody has mentioned airflow across the condenser at any point in this discussion. At 2000 RPM without the car moving, the pressures will be elevated due to lack of airflow across the condenser. For any meaningful measurement, you should have a fairly strong fan blowing across the condenser when doing your measuring. At 105˚F, the pressures are going to be high anyway. They'll be higher than when the car is moving, even with a strong fan blowing across the condenser.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]

This is the right way to do things. That said, a minute quantity of moisture is not going to noticeably affect performance of the system. I would be skeptical that moisture is in any way responsible for the high head pressure/low performance of your system.

I am well aware of 123 vs. 124, have owned and fixed both! Different Denso compressors are used in different 124 models as well. I saw quite different performance in an early diesel (10p15 compressor, R12, single electric condenser fan and low idle RPM) vs later gasoline model (10p17 compressor, R134a, dual electric condenser fans, somewhat higher idle speed).

To be clear, the 30F drop is the temperature delta of the air into the evap vs. air out of the evap in a single pass with fan on maximum speed. I'm not talking about discharge air temp relative to ambient temp after the fan has slowed down - that measurement really doesn't prove anything because there are too many variables.

My '84 300D would discharge ~48F air on a humid 100F+ day in Dallas - at highway speed, after the cabin had cooled down a bit, and with the 100% recirc mod. Should have more completely specified the parameters around my statement, I suppose.

TRUE.
Professional shops will usually have something like this on wheels so they can put it right up against the grill...

https://sc01.alicdn.com/kf/HTB1RQSjNpXXXXctXVXXq6xXFXXXi/3000-cfm-centrifugal-blower-fan-high-pressure.jpg

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1980 240d ,

chain elongation, cam marks reference:
http://www.peachparts.com/shopforum/diesel-discussion/10414-help-i-need-check-stretch.html

http://www.peachparts.com/shopforum/diesel-discussion/305365-9-degrees-chain-stretch.html

evap fin cleaning:

http://www.peachparts.com/shopforum/diesel-discussion/156207-photo-step-step-post-showing-w123-evaporator-removal-1983-240d-1982-300td.html?highlight=evaporator

A/C thread

http://www.peachparts.com/shopforum/diesel-discussion/297462-c-recommendations-mb-vehicles.html

This is the problem I have when I post this issue online, everybody just reads the pressure, but does not read that I am UNDERCHARGED. I need someone who has more experience to tell me whether an overcharge of oil will provide excess pressures. That is all! If excess oil will not do this, then I can look elsewhere!

__________________
2016 Monsoon Gray Audi Allroad - 21k
2008 Black Mercedes E350 4Matic Sport - 131k
2014 Jeep Wranger Unlimited Sahara - 62k
2003 Gray Mercedes ML350 - 122k

is the electric fan at the condenser blowing wide open during the time you are getting the hi reading?

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Jim

Have you bothered to deal with airflow across the condenser? If not, your pressures are arbitrary and meaningless. The system pressures will continue to vary with load and with the amount of refrigerant in the system. Even if you keep adding refrigerant, the head pressure isn't likely to rise very quickly as the liquid begins to fill the condenser.

Charge by weight (80% of the R12 weight) and THEN check pressures with a fan blowing across the condenser. At 105˚ and a high load on the system, your pressures will be high. My SDL runs ~350-360/55-60 at the same temps with the fan on high and all 4 windows down. Once the cabin pulls down to temp, the pressures drop dramatically, more along the lines of 250-275/30-35.

Load and airflow matter when measuring pressure. This is why you charge by weight.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]